Many integrated circuit devices are now used in electrical appliances. Many integrated circuit devices used in the electrical appliances are so-called “ASIC (Application Specific Integrated Circuit)” and dedicated circuits designed for particular electrical appliances. In these ASICs, because a cell (logic circuit such as an AND circuit and an OR circuit) is arranged and interconnected in manufacturing steps, a circuit configuration thereof can not be changed after manufacturing.
Recently, competition to develop electrical appliances has intensified and miniaturization continues to make progress. In these circumstances, a programmable logic circuit in which a circuit configuration thereof can be changed by an electric signal even after manufacturing and which can realize many functions on single chip draws interest. A representative example of a programmable logic circuit is a FPGA (Field-Programmable Gate Array) or a DRP (Dynamically Reconfigurable Processor) etc.
The programmable logic circuit attracts attention because of this feature, but implementation of a programmable logic circuit to an electric appliance has up to now been limited. The reason is as following.
That is, in a conventional programmable logic circuit, there have been presented problems that a size of a switching device for interconnecting a logic cell (i.e. a unit logic circuit for assembling the programmable logic circuit—the programmable logic circuit is configured by interconnecting a plurality of the logic cells using the switching device) is large, and on-resistance thereof is high. Then, in order to reduce the number of switching devices that have a large size and high on-resistance as much as possible, the conventional programmable logic circuit is configured in a manner that logic cells having as many transistors as possible are used to reduce the number of logic cells and the number of switching devices which interconnect the logic cells. As a result, the degree of freedom in combining logic cells was decreased, thus limiting functions which the programmable logic circuit could provide. That is, the size and the high on-resistance of the switching device that was used in the programmable logic circuit, limited functionality of the programmable logic circuit, thus limiting implementation of the programmable logic circuit in electric appliances.
Then, in order to diversify functionality of the programmable logic circuit and promote implementation in electric appliances, it is necessary to reduce the size of the switching device for interconnecting logic cells with each other and to decrease its on-resistance.
A switching device satisfying these requirements, which uses the conduction phenomenon of metal ions in an ion conductor (solid inside of which ions can freely move) and electrochemical reaction, has been proposed (hereinafter, called “metal atom migration switching device”). (For example, see Japanese Patent Laid-Open No. 2002-076325 and National Publication of International Patent Application No. 2002-536840).
It is known that a metal atom migration switching device is smaller than a semiconductor switching device that (that example, a MOSFET) is used often in conventional programmable logic and has a lower on-resistance. This metal atom migration switching device is widely divided into two types shown in FIGS. 1A and 1B.
FIG. 1A shows a metal atom migration switching device with a gap, and FIG. 1B shows a metal atom migration switching device without a gap. The metal atom migration switching devices shown in FIGS. 1A and 1B both are metal atom migration switching devices having two terminals.
The metal atom migration switching device with a gap shown in FIG. 1A (see Japanese Patent Laid-Open No. 2002-076325) is a metal atom migration switching device having two terminals which include an ion conducting part composed of an ion conductor (Ag2S), a second electrode (Ag) for supplying metal ions (Ag+) to the ion conducting part, or for receiving the metal ions (Ag+) from the ion conducting part to precipitate metal (Ag) corresponding to the metal ions and formed to be in contact with the ion conducting part, and a first electrode (Pt) formed to have a gap with the ion conducting part. (Material shown FIG. 1A and for each component described above is exemplary).
When a negative voltage relative to the second electrode (Ag) is applied to the first electrode (Pt) as shown in FIG. 1A, electrons, after penetrating an energy barrier (tunneling) in the gap between the first electrode (Pt) and the ion conducting part, reach the surface of the ion conducting part from the first electrode (Pt) and reduce the metal ions (Ag+) near the surface of the ion conducting part, precipitating metal (Ag).
When the metal (Ag) is precipitated, in response to this, metal (Ag) in the second electrode is oxidized to melt in the ion conducting part as metal ions (Ag+), and thereby, balance between positive ions and negative ions in the ion conducting part is maintained. When the precipitated metal (Ag) on the surface of the ion conducting part grows to be in contact with the first electrode (Pt), the switching device enters a conduction (on) state (see the left drawing in FIG. 1A).
On the one hand, when a positive voltage relative to the second electrode (Ag) is applied to the first electrode (Pt), quite the reverse electrochemical reaction occurs. As a result, the precipitated metal (Ag) leaves the first electrode (Pt) and the switching device enters a non conduction (off) state (see the right drawing in FIG. 1A). In addition, as an ion conductor, a semiconductor or an insulator (for example, Ag2S is an n-type semiconductor) may be used. However, in order to operate as a switching device (to provide a conduction (on) state), it is desirable for the ion conductor to have a large contact area with the second electrode and be formed comparatively thinly.
The metal atom migration switching device without a gap shown in FIG. 1B (see National Publication of International Patent Application No. 2002-536840) is a metal atom migration switching device having two terminals which includes an ion conducting part composed of an ion conductor (Cu2S), a second electrode (Cu) for supplying metal ions (Cu+) to the ion conducting part, or for receiving the metal ions (Cu+) from the ion conducting part to precipitate metal (Cu) corresponding to the metal ions, and which is formed to be in contact with the ion conducting part, and a first electrode (Ti) formed to be in contact with the ion conducting part (Material shown in FIG. 1B and for each component described above is exemplary).
When a negative voltage relative to the second electrode (Cu) is applied to the first electrode (Ti) as shown in FIG. 1B, the metal ions (Cu+) near the contact surface between the ion conducting part and the first electrode (Ti) are reduced, precipitating metal (Cu) on the contact surface between the ion conducting part and the first electrode (Ti). When the metal (Cu) is precipitated, in response to it, metal (Cu) in the second electrode is oxidized to melt in the ion conducting part as metal ions (Cu+), and thereby, balance between positive ions and negative ions in the ion conducting part is maintained.
Generally, because the ion conductor (Cu2S) of the ion conducting part is softer than the first electrode (Ti), the precipitated metal (Cu) grows toward the second electrode (Cu) in the ion conducting part. When the precipitated metal (Cu) comes in contact with the second electrode (Cu), the switching device enters to a conduction (on) state (see the left drawing in FIG. 1B).
On the other hand, when a positive voltage relative to the second electrode (Cu) is applied to the first electrode (Ti), quite the reverse electrochemical reaction occurs. As a result, the precipitated metal (Cu) leaves the second electrode (Cu) and the switching device enters a non conduction (off) state (see the right drawing in FIG. 1B).
The two types of metal atom migration switching devices shown in FIGS. 1A and 1B have differences in configuration and operation described above, but in both of them, due to the electrochemical reaction, the metal atoms in the second electrode move between the first electrode and the second electrode as a precipitate to form a metal wire for making a connection between the first electrode and second electrode (when in the conduction (on) state).
The two types of metal atom migration switching devices shown in FIGS. 1A and 1B both are metal atom migration switching device each having two terminals. Such a metal atom migration switching device having two terminals has a problem of low electro-migration resistance.
Electro-migration means a phenomenon in which metal atoms collide with electrons, that are flowing in the metal wires, and which are to be moved in metal wires. In high temperature environments, the flow of electric current having a current density over a certain level is maintained in a metal wire, due to movement of the metal atoms caused by the electro-migration and a serious problem such as breakage of metal wires will occur.
As described above, in a metal atom migration switching device having two terminals, metal atoms, after moving from the second electrode to the first electrode as a precipitate due to the electrochemical reaction, form a metal wire to connect between the first electrode and the second electrode. To prevent electro-migration in this metal wire, it is necessary to increase the amount of precipitate to form a thicker metal wire and to decrease the electric current density of current flowing in the metal wire.
However, in the metal atom migration switching device having two terminals, it is not easy to increase the amount of precipitate to make the metal wire thicker. This is because, to increase the amount of precipitate, it is necessary to increase the absolute value of the negative voltage, relative to the second electrode, that is applied to the first electrode. However, once the metal wire for making a connection between the first electrode and the second electrode is formed, the voltage applied between the first electrode and the second electrode may contribute to causing a large amount of electric current to flow, but it does not contribute to increasing the amount of precipitate to make the metal wire thicker. On the contrary, if the voltage is increased to prevent electro-migration, a large amount of current flows in the metal wire, and thereby, more electro-migration may be induced.